(i) Field of the Invention
The invention relates to systems for providing cathodic protection to metals and alloys subject to corrosion when in contact with electrically conductive and corrosive liquids. A preferred embodiment of the invention resides in a system for protecting from corrosion large metal holding tanks containing a super-cooled aqueous solution of calcium chloride brine used for rapidly cooling and/or freezing of poultry.
(ii) Description of the Related Art
Many metals and alloys, particularly those comprising a significant iron content, are known to corrode or rust when exposed to salt water or other environment electrolytes capable of conducting and transferring an electric current, and thereby transporting ions from the metal. To retard corrosion of such metals, it is known to apply anodic protection or to apply coatings, and/or to apply cathodic protection. Cathodic protection is particularly used with pipes, pumps, heat exchangers, holding tanks, and other containments of aqueous solutions where corrosion would occur in the absence of cathodic protection.
FIG. 1 is a schematic diagram depicting a well-known prior art system with impressed current providing cathodic protection to a corroding metal 8 immersed in an aqueous solution. If cathodic protection were not provided, surfaces of the metal would act as local cathodes 12, while other surfaces would act as local anodes 14. In such an arrangement, potential differences would arise between the anodic and cathodic surfaces due to their exposure to different solutions and/or metal chemistries transferring current in the conductive solution.
Cathodic protection of the corroding metal 8 can be accomplished by coupling the negative terminal of a voltage and current source 10 to the metal with a corroding metallic surface. An auxiliary anode 16 electrically coupled to the positive voltage terminal of the voltage and current source impresses electrical current from the auxiliary anode to both the cathodic and anodic surfaces of the corroding metal before returning to its source. The current is impressed until the entire surface of the corroding metal polarizes toward almost the same potential, thereby preventing electrical current from transferring between different surface exposures on the metal. Accordingly, the metal should not corrode so long as the external current is maintained, because the positively charged cations travel in one direction through the aqueous solution toward the cathode or the metal surface being protected, whereas negatively charged anions, including corrosive ions travel toward the anode.
Cathodic protection can also be employed to counteract and stifle microbiologically influenced corrosion (MIC). Strict (or obligate) anaerobes, in particular sulfate reducing bacteria (SRB), such as Desulfovibrio desuluricans, accumulate and function in the absence of oxygen under deposits and produce H2S, which produces an unpleasant odor, and in combination with iron, forms iron sulphide. In addition, carbon dioxide and hydrogen (produced by cathodic protection) are consumed by methane-producing bacteria methanogens which often coexist in a symbiotic relationship with SRB; thus, these bacteria are capable of promoting cathodic depolarization.
Many aerobic bacteria form sticky slime of extracellular polymers on stainless steels and other metallic surfaces which are ideal sites being devoid of oxygen for SRB. Aerobic bacteria, such as thiobacillus strains produce acids which oxidize sulphide and sulfur forming sulfuric acid as a metabolic by-product under anaerobic deposits where they are usually accompanied by SRB. Also, where iron, manganese, and chlorides are present with iron oxidizers or aerobes, such as Gallionella bacterium, ferric-manganese chloride is produced thereby promoting potent pitting into stainless steels. Moreover, as precipitation of deposits may either be induced or be inhibited through the use of pH control in conjunction with cathodic protection and through bacteria such as SRB (which may still thrive in highly alkaline solutions), deposits should be avoided so that the targeted pH value of the protective film at a steel interface should remain above 10.
Cathodic protection has been used in connection with stainless steel containers holding super-cooled liquid used to rapidly freeze food products such as fowl. Such chillers or freezers typically include an impressed current and voltage source with anodes immersed in the metallic holding tank containing an aqueous liquid solution cooled toward or below the freezing point of water. While such cathodic protection systems have been shown to reduce or prevent corrosion in metallic piping, pumps, and/or of holding tanks (which would otherwise periodically need to be replaced, thereby halting the freezing process), impressed current protection systems often produce undesirable side effects such as the evolution or emission of oxygen and/or chlorine; chlorine production is an environmental safety hazard and also results in the production of corrosive hydrochloric acid. In addition, these systems often need to be recalibrated due to shifting potentials at the surfaces to be protected. Moreover, where seawater is used for cooling, magnesium chloride (a natural constituent of seawater) hydrolyzes into hydrochloric acid, which may corrode components of the pumps, tanks, piping, and heat exchanger cooling equipment.
It would be desirable, therefore, to provide a cathodic protection system that is selfcalibrating and that ensures sufficient protective current is applied to produce a highly alkaline protective film to preserve a metallic structure in a state of immunity without the potentially unsafe and undesirable side effects that have been present in known systems. The present invention satisfies this and other needs and provides further related benefits and advantages.
It would be particularly desirable to provide a cathodic protection system to preserve metallic structures for use with the rapid chilling and/or freezing of food products such as poultry including whole muscle turkeys.
The current invention is embodied in a system for the cathodic protection of a wetted and/or immersed surface of a metal structure containing or in contact with an electrolyte comprising a voltage and current source impressing current from an electrically coupled anode. A non-metallic chamber contains an electrically coupled anode immersed in an anolyte. A cation exchange membrane impermeable to Cl ions serve as a barrier to separate the anolyte from contacting the electrolyte, but allows protective current transfer by migration of cations from the anolyte along with water into the electrolyte and at the cathode. This separation permits cathodic protection of the metallic surface without any of the adverse side effects accompanying conventional cathodic protection systems in this application.
In one embodiment, the current invention is used to cathodically protect the internal surface of stainless steel pumps, piping, heat exchangers, and holding tanks used in connection with the provision of a low temperature bath to freeze whole muscle turkeys. Turkeys packaged for retail sale are chilled or frozen within a calcium chloride brine bath cooled by a heat exchanger immersed within the holding tanks. The brine is slowly circulated through the holding tanks by the action of pumps causing the floating turkeys to rapidly cool and/or freeze until they reach a far end of the holding tank where they are removed by a conveyor. This process may be repeated, if necessary. Along each holding tank are a series of anode chambers, each having an anode, an anolyte, and a cation exchange membrane acting as an interface separating the anolyte from the brine, thereby preventing the anodic production of Cl2; the impressed current source couples the anode and the stainless steel structures being cathodically protected.
Some embodiments also include auto-potential controllers coupled to reference electrodes to monitor potential differences between the electrodes and the metallic surfaces being cathodically protected. If the potential difference and/or exposure conditions fall outside of a predetermined range, the voltage level to the anodes impressing current is accordingly adjusted to counteract and overcome the corrosive properties of the brine by producing protective film over the cathodic surfaces.
The current invention is also embodied in a system for rapidly chilling fowl products comprising a holding tank having a first end and a second end containing a chilled aqueous bath circulating from the first end of the holding tank to the second end of the holding tank. Means of conveyance for the fowl products into and out of the bath are provided along with a safely separated means of applying protective current for the cathodic protection process to prevent the holding tanks and associated equipment from corroding.
Though the cathodic protection system herein disclosed is used in conjunction with the rapid cooling or freezing of food products, it will be understood that such a system can be applied to any corrosive electrolytic environments involving metals compatible with cathodic protection and associated processes such as underground piping, on-grade tank bottoms, desalination equipment, cooling water tanks and piping, heat exchangers, pumps, feed bins and corn bins, and equipment for beverage production.